The present invention relates generally to a mechanical power transmission for driving a vehicle, which transmission conveys the driving force output from the engine to the drive wheels of the vehicle with a speed change being performed by a speed change mechanism.
Many such transmissions have been used for driving vehicles, and a variety of speed change mechanisms are incorporated in the transmissions, ranging from gear transmissions with a plurality of steps of speed change ratios to continuously variable transmissions with gradually varying speed change ratio controlled by means of a belt, etc. Recently, a hybrid transmission which uses an electrical motor in addition to an engine has been introduced for fuel efficiency. The applicant of the present invention is also developing a hybrid transmission. This hybrid transmission comprises a continuously variable transmission with a metal V-belt which is disposed around the common output shaft of the engine and the electrical motor arranged in series, and the output shaft of the continuously variable transmission is provided with a starting clutch (main clutch).
One purpose for developing this transmission is to improve fuel efficiency. Therefore, desirably, the operation of the engine be controlled such that the engine stops its operation when the vehicle come into a halt (such control is here referred to as xe2x80x9cidling elimination controlxe2x80x9d). As an idling elimination control, there is a method to stop the engine when the engine comes into an idling condition after the vehicle has stopped completely. However, while the vehicle is decelerating after the accelerator pedal once depressed has been released, an engine brake is effected. It is known that, during this deceleration, the fuel injection to the engine is terminated (or the supply of fuel is terminated). In consideration of this fuel supply termination, there is a more desirable way to eliminate engine idling. When the vehicle is decelerated to a halt, the fuel injection is terminated, and this condition of non-fuel supply should be maintained to stop the operation of the engine (engine strop control).
In general, the oil pump which generates the hydraulic pressure to control the transmission is connected to the output shaft of the engine (i.e., the input shaft of the speed change mechanism) and is driven by the engine when the engine rotates. Also in the above mentioned hybrid transmission, which comprises an engine and an electrical motor disposed in series, the oil pump is connected to the common shaft (i.e., the input shaft of the speed change mechanism) of the engine and the electrical motor, and it is driven by the engine or the motor as either rotates. Therefore, when the vehicle comes into a halt, if the engine stops, the hydraulic pressure decreases to zero. When the engine or the electrical motor (these elements are hereinafter referred to as xe2x80x9cengine, etc.xe2x80x9d) is restarted to start the vehicle again, the pressure is regenerated.
Here, if the temperature of the oil, which is the medium used for controlling the engagement and disengagement of the starting clutch (main clutch), is sufficiently high, then the pressure, which has once decreased, quickly returns to a normal level enabling the system to control every part as necessary when the engine, etc. are restarted. However, if the temperature is low, then there is a tendency that the pressure of the hydraulic circuit is not easily released after the engine is stopped or not easily returned to a sufficient level after the engine is restarted because of a high viscosity of the oil (i.e., the viscous resistance is great).
From this reason, the following problem is likely to occur. When the temperature of the oil is low, the operation of the starting clutch cannot follow each command signal received in the engagement control of the starting clutch and lags to engage the clutch discs (i.e., a starting up lag). Generally, the starting clutch is controlled such that the clutch discs are quickly brought close to each other to clear the part of the stroke which does not contribute to the actual engagement of the clutch for the purpose of saving time in the control of the starting clutch (this is referred to as xe2x80x9cinvalid stroke clearing controlxe2x80x9d). If the temperature of the oil is low, the pressure necessary for performing the invalid stroke clearing control does not become available immediately. As a result, as the pressure for the engagement control increases, the engagement of the clutch may progress rapidly or suddenly in a runaway fashion and may generate a shock.
It is an object of the present invention to provide a stop control system for a vehicular transmission which system alleviates the above mentioned starting up lag and shock and shortens the time during which such phenomena may take place.
In order to achieve this objective, the present invention provides a stop control system for a mechanical power transmission used for driving a vehicle. Here, the power transmission comprises a speed change mechanism (for example, the continuously variable transmission CVT of the embodiment described in the following section), which conveys a driving force from an engine through frictionally engaging means (for example, the starting clutch 5 of the following embodiment). In this transmission, the stop control system controls the termination of the operation of the engine when the vehicle is brought into a halt. The control system allows the termination of the operation of the engine only when the temperature of the hydraulic oil used for generating the hydraulic pressure that controls the engagement and disengagement of the frictionally engaging means is equal to or higher than a predetermined temperature.
The stop control system according to the present invention allows the engine to cease its operation when the temperature of the oil used for the engagement and disengagement of the frictionally engaging means is equal to or higher than a predetermined temperature (i.e., a temperature which endows the oil a viscosity suitable for the actuation control of the starting clutch, for example, 20 degrees Celsius). When the oil temperature is low, the engine is not allowed to stop its operation even though the vehicle is brought into a halt. In this way, the hydraulic pressure supplied from an oil pump to the hydraulic circuit which controls each hydraulic actuator of the transmission is kept at a pressure high enough for the operation of the transmission, so a prevention is made against the above mentioned problems of start up lag and start up shock. Even if the oil temperature becomes lower while the vehicle is not driving, because the engine is kept in operation by this control system, the time during which a start up lag and a start up shock may occur is shortened effectively, so the hydraulic pressure returns to a suitable level quickly.
It is preferable that the pressure of the oil used for actuating the transmission be raised when the oil temperature is lower than the predetermined temperature. To raise the hydraulic pressure, for example, the hydraulic pressure generating the lateral thrust applied to the drive and driven pulleys for varying the speed change ratio of the metal V-belt continuously variable transmission can be increased, the hydraulic pressure used for actuating a forward clutch or a reverse brake which is used for switching the direction of the drive of the vehicle can be increased, the hydraulic pressure used for maintaining the starting clutch engaged after the engagement of the clutch can be increased, or the pressure of the oil discharged from the oil pump can be increased. Taking one of such measures promotes a temperature increase of the oil. For example, if the lateral thrust is increased, then the friction generated between the grooves of the pulleys and the metal V-belt increases correspondingly. As a result, more frictional heat is generated, contributing to the increase of the temperature of the oil. Also, if the discharge pressure of the pump is increased, then the heat generated by the pump is increased, so the temperature increase is also promoted. In this way, the oil temperature is raised quickly to shorten the time during which the above mentioned unfavorable phenomena may take place.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.